Each NDR provides a 3-digit status notification code that defines the failure in detail. Each of these three digits provides information about the failure itself and can be translated as follows:
2.x.x > successful transfer
4.x.x > persistent failure
5.x.x > permanent failure
The following list describes second and third digit:
x.1.0 > other address status
x.1.1 > bad destination mailbox address
x.1.2 > bad destination system address
x.1.3 > bad destination mailbox address syntax
x.1.4 > destination mailbox address ambiguous
x.1.5 > destination mailbox address valid
x.1.6 > mailbox has moved
x.1.7 > bad sender’s address syntax
x.1.8 > bad sender’s system address
x.2.0 > other or undefined mailbox status
x.2.1 > mailbox disabled, not accepting messages
x.2.2 > mailbox full
x.2.3 > message length exceeds administrative limit
x.2.4 > mailing list expansion problem
x.3.0 > other or undefined mail system status
x.3.1 > mail system full
x.3.2 > system not accepting network messages
x.3.3 > system not capable of selected features
x.3.4 > message too big for system
x.4.0 > other or undefined network or routing status
x.4.1 > no answer from host
x.4.2 > bad connection
x.4.3 > routing server failure
x.4.4 > unable to route
x.4.5 > network congestion
x.4.6 > routing loop detected
x.4.7 > delivery time expired
x.5.0 > other or undefined protocol status
x.5.1 > invalid command
x.5.2 > syntax error
x.5.3 > too many recipients
x.5.4 > invalid command arguments
x.5.5 > wrong protocol version
x.6.0 > other or undefined media error
x.6.1 > media not supported
x.6.2 > conversion required and prohibited
x.6.3 > conversion required but not supported
x.6.4 > conversion with loss performed
x.6.5 > conversion failed
x.7.0 > other or undefined protocol status
x.7.1 > delivery not authorized, message refused
x.7.2 > mailing list expansion prohibited
x.7.3 > security conversion required but not possible
x.7.4 > security feature not supported
x.7.5 > cryptographic failure
x.7.6 > cryptographic algorithm not supported
x.7.7 > message integrity failure
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